Image formation by an electrophotographic method includes developing an electrostatic image with a toner to visualize the image, transferring the toner image thus obtained onto a sheet, and then fixing the toner image by applying heat and pressure thereto. The toner is produced by melting and kneading a mixture containing a binder resin, a colorant, and a charge control agent or the like, and grinding and classifying the resultant mixture to adjust the particle size distribution. Petroleum resins such as a styrene-acrylic resin and a polyester resin are conventionally used as the binder resin of the toner.
In recent years, use of a biodegradable resin having a small load on the environment upon disposal or a biomass plastic made from a recyclable resource as a resin for toners has been proposed from the standpoint of environmental friendliness. Biomass plastics and biodegradable plastics which can effectively utilize limited resources and contribute to a reduction in an environment load are called bioplastics.
At present, one of the most promising bioplastic resins is polylactic acid. The polylactic acid is a crystalline polyester having a melting point of about 170° C., a glass transition point of about 60° C., and a weight average molecular weight (Mw) of about 100,000 to about 150,000. The use of such polylactic acid to which heat resistance and high durability are imparted has also been started as a material of a casing of a mobile phone or the like.
However, when the above polylactic acid is used alone as a toner resin, it is disadvantageously hard, causing poor grindability, and has a disadvantageously high softening temperature, thus is not suitable for low-temperature fixing.
Jpn. Pat. Appin. KOKAI Publication No. 2003-248339 describes that good low-temperature fixability is attained without impairing durability by blending a specific polylactic acid-based biodegradable resin with a terpene-phenol copolymer and a predetermined amount of wax having a melting point equal to or lower than the softening temperature of the resin. However, the use of polylactic acid described in the Examples of the gazette makes it difficult to attain high grindability.
Jpn. Pat. Appin. KOKAI Publication No. 2014-041324 describes that a phenol-based condensation product or a phenol resin is used as a charge control agent in a toner containing a biodegradable resin as a binder resin. However, the gazette does not refer to grindability in a toner producing process.
Jpn. Pat. Appin. KOKAI Publication No. 2012-032628 proposes that polylactic acid is hydrolyzed to have a smaller molecular weight so as to improve grindability. However, the hydrolysis of the polylactic acid requires a long processing time, which causes a great burden in terms of workability and cost.